1. Field of the Invention
The present invention relates to display apparatuses such as a display having a plurality of light-emitting diodes (LEDs). More particularly, the invention relates to a high quality LED display apparatus having a relatively simple structure which provides superior image contrast properties.
2. Discussion of the Related Art
In recent years, there have been developed light-emitting diodes capable of emitting red, green, and blue colors (hereinafter referred to as “RGB” in some cases) and light-emitting diodes which are formed of light emitting elements in combination with phosphor materials so that they emit various colors, such as a white color, with a high brightness. Since a light-emitting diode (hereinafter referred to as an “LED”) is a solid-state element, it has superior resistance against vibration, a low-power consumption rate, a long operating life, and superior drive properties such as a high-speed drive. Accordingly, the light-emitting diodes have been increasingly used for various applications such as light sources for signal heads and full-color LED displays.
When the LED display apparatuses described above are used, a black or frosted mask member is provided between individual light-emitting diodes arranged on an observation side of a circuit board in order to increase the contrast. The mask member described above is made of a resin material or the like. The mask member, in the form of a plate having openings provided therein, is placed on the circuit board. The openings are located at predetermined positions in the mask member so that the light-emitting diodes on the circuit board penetrate or fit through the respective openings. In addition, since the mask member is provided on the circuit board, the mask member also serves as a protective layer for protecting the conductive pattern.
The mask member described above must be formed to have a dimensional tolerance to a certain extent since the light-emitting diodes are arranged on the circuit board and should penetrate through the respective openings. The openings are each formed slightly larger than the size of the light-emitting diode to consider many features including the contrast, the thermal expansion of the light-emitting diodes during light emission, and the like.
The technique described above has been disclosed, for example, in Japanese Unexamined Patent Application Publication No. 10-161569.
However, when the mask member as described above is provided on the circuit board, dust and the like may enter spaces between the openings and the light-emitting diodes and may gradually accumulate on the circuit board. Also it becomes difficult to perform cleaning due to the presence of dust at the boundaries between the openings and the light-emitting diodes. In addition, operational malfunctions may occur in some cases. An LED display apparatus used indoors usually does not use a cover or the like on the observation side, and hence in the case described above, it becomes more difficult for the mask member to protect the circuit board from dust and the like. In addition, a degradation in the contrast ratio which is caused by dust and the like adhering to the observation-side surface of the LED display apparatus should be prevented.